One unresolved issue around marijuana legalization has been how to keep stoned users from getting behind the wheel. This week, Colorado Democrats Jared Polis (CO-02) and Ed Perlmutter (CO-07) began to remedy that by co-sponsoring the Limiting Unsafe Cannabis-Impaired Driving (LUCID) Act.

If passed, the law would include marijuana in the federal definition of impaired driving; it would make access to federal highway funding in states that have legalized recreational or medical marijuana contingent upon those states having laws against marijuana-impaired driving and methods for enforcing them; and it would mandate that the National Highway Traffic Safety Administraton (NTHSA) do the necessary testing and research on marijuana and driving to help states determine the most effective means of enforcement.

The last point is the key to the whole thing, and it’s the primary difference between this year’s LUCID Act and a similar law Polis proposed in 2014. Virtually everyone agrees that treating marijuana like alcohol logically steers us toward the creation of laws that address driving while stoned. The problem is, research on the subject is still so thin that no one can say definitively what amount of weed usage impairs a driver, or what the best way is to test for it.

Past legislative proposals would have set the limit of marijuana intoxication at five nanograms per liter of blood, which could be determined by a blood test. Of course, administering such tests on the side of the road or even at a police station presents obvious complications. Moreover, the five nanogram limit becomes unenforceably arbitrary when you conisder how long THC can linger in your system. For example, someone who got high last night still could have more than five nanograms in his blood the following day—or even the day after that—despite being completely sober.

That’s why much more testing is needed before we have a breathalyzer-style roadside test as reliable as the ones we have for alcohol. But with momentum trending in favor of marijuana legalization nationwide, a number of ancillary laws are getting more love from legislators. The LUCID Act would be a positive step toward one of the most important safety regulations we must have in place for legalization to be worthwhile.